The foam heretofore known includes polyurethane, polyurea, polystyrene, polyolefin, and polycyclopentadiene foams. In addition, as an innovation conducive to improved productivity of polyolefin foam, an ionomer foam produced by reacting a carboxyl group-containing copolymer with a metal carbonate (Japanese Kokoku Publication Hei-2-11621) has been proposed.
However, the conventional foams have several drawbacks: use of raw materials unfriendly to the environment, evolution of noxious gases and soot due to incomplete combustion, excessive generation of heat of combustion during incineration which detracts from the serviceable life of the incinerator, and generation of static electricity. The ionomer foam is also disadvantageous in that the starting carboxyl group-containing copolymer is either a solid or a super-high-viscosity liquid which requires a melt-kneading extruder capable of high temperature and high pressure services and that the expansion ratio is as small as about 2 to 5, thus failing to provide only a low-density foam. Furthermore, when the carboxyl group-containing copolymer is dissolved in water or an organic solvent to reduce its viscosity and fed to a low-pressure foaming machine, the insufficient foam strength tends to result in collapse of cells, thus failing to give a foam of low density. In addition, because the density of the foam is not sufficiently low, the temperature-time area and the emitting smoke factor are markedly increased in the test for incombustibility (JIS A-1321) so that the requirements for flame retardancy grades 1 to 3 are not satisfied.
The inventors of the present invention explored into low-density foams that would be benign to the environment and could be easily produced, for example using a low-pressure foaming machine. The present invention is the result of this research endeavor.